There seems to have been an explosion of Christian heresy in medieval Europe, but studying the phenomenon has always been devilishly tricky.

Challenging questions confront historians. Was there really more heresy, or were the ecclesiastical and secular authorities simply more alert – or paranoid? Were there coherent heretical ‘movements’ or did those aforementioned authorities impose an artificial unity on their religious foes to make them appear more menacing?

Finally, and this is perhaps the greatest hurdle, how is the historian to approach the scant and usually biased sources?

Almost all the important accounts were penned by hostile witnesses and often at some chronological distance from the heretical events in question. It is an interpretative minefield but Robert Moore has been treading carefully through it for many decades. His latest book is as good, and as provocative, as anything he has produced.

The crucial message is that we should not take anything for granted.

When analysing a medieval heresy we should look closely at the surviving textual evidence. When was it written? Why was it written?

Such painstaking examination reveals a befuddling historical landscape. The identification and condemnation of heresy was primarily a theological pursuit but it was frequently influenced by political and intellectual rivalries. The difference between acceptable and disruptive religious behaviour was often in the eye of the beholder or defined by historical happenstance.

Most importantly of all, we should be sceptical of neat and tidy terms and definitions. Everyone, for instance, has heard of the Cathars but we should recognise that the individuals involved had divergent views.

There is a huge risk of replacing the historical reality with a convenient construct. This made good strategic sense to medieval inquisitors but 21st-century historians should be wary of following suit.

There is a clear and admirable historiographical agenda at the heart of this book. Moore wants historians to improve their methodology and, winningly, he does not exempt his own earlier writings from criticism. The book is much more than a sermon, however. It is also one of the finest accounts of medieval heresy that you are likely to encounter.

All of the famous (and not so famous) heretical figures are here, the emergence of inquisitorial tribunals is neatly explained, and the broader socio-economic context is explored.

Many of the riddles of medieval heresy will never be solved because we lack sufficient evidence but Moore’s book brings us closer to an unachievable goal: explaining why something very odd happened in Christendom between the 12th and 14th centuries.

We can dismiss many of the contemporary conspiracy theories about an organised, continent-wide heretical threat. There were at least as many dreamed-up bogeymen as genuine heresiarchs. But, with all caveats considered, we still have to recognise that religious dissent spiralled during this period.

What were the causes? The contenders include anxiety in a rapidly changing world, a genuine quest to return to apostolic purity, grumblings about stalled efforts at church reform, and a high incidence of unique religious odysseys.

There were brutish heretical demagogues who took advantage of the situation and well-intentioned spiritual seekers who were tarred with the heretical brush. If we add the difference between actual heresy and the self-serving fantasies of lofty medieval heresy-bashers to the mix, the result is quite a muddle.

Overarching interpretations are very tempting. Moore has not always resisted the temptation but here he encourages us to pull the texts apart and ask specific questions beginning with words like what, why and when.

That’s a very good idea and one that serves to enhance Moore’s status as one of the finest historians of medieval heresy.

Jonathan Wright is the author of Heretics: The Creation of Christianity from the Gnostics to the Modern Church (Houghton Mifflin Harcourt, 2011)

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